1. Field of the Invention
This invention relates to a photo detecting apparatus for detecting light emitted or reflected by a scanning surface, which is scanned with a light beam. This invention particularly relates to a photo detecting apparatus, wherein the emitted light or the reflected light is propagated through a light guide member and is thereby guided to a photodetector.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor,
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed, and the processed image signal is used during the reproduction of a visible image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In the radiation image recording and reproducing systems described above, operations for detecting the light emitted by stimulable phosphor sheets are carried out with one of two methods described below.
In one of the methods, division into picture elements is carried out by the scanning with stimulating rays, and the light emitted by a stimulable phosphor sheet is detected by a light receiving device having a wide light receiving face (for example, a photomultiplier or a photoconductive device). In the other method, division into picture elements is carried out by a light receiving device (for example, a two-dimensional solid state image pickup device or a semiconductor line sensor), and a serial image signal is generated by an electric circuit.
The former method is more advantageous in the sensitivity, the signal-to-noise ratio (S/N ratio) of the read-out signal, or the like, than the latter method using the two-dimensional solid state image pickup device, the semiconductor line sensor, or the like. However, the light emitted by the stimulable phosphor sheet is non-directional and has a low intensity. Therefore, when the former method is carried out, the emitted-light receiving solid angle should be set as large as possible such that much light may be collected and the condensing efficiency can thereby be kept high. If the condensing efficiency is low, the S/N ratio of the read-out signal becomes low and, in the worst case, the readout of the image stored on the stimulable phosphor sheet becomes impossible.
Therefore, special light guide members have heretofore been used widely in order to improve the condensing efficiency for the light emitted by the stimulable phosphor sheet. As disclosed in, for example, Japanese Unexamined Patent Publication No. 56(1981)-12601 and U.S. Pat. Nos. 4,346,295 and 4,485,302, the light guide member is made from a light guiding material, such as an acrylic resin. The light guide member has a flat light input end face, which is positioned so that it extends along a main scanning line of stimulating rays on a stimulable phosphor sheet. The light guide member is rounded little by little from the side of the light input end face towards a cylindrical light output end face. The light output end face is positioned so that it may be connected to a light receiving face of a photodetector. In cases where such a light guide member is used, the light input end face can be located close to the stimulable phosphor sheet, and much light emitted by the stimulable phosphor sheet can thereby be caused to impinge upon the light input end face. Also, the emitted light, which impinges upon the light input end face of the light guide member can be efficiently guided to the light receiving face of the light receiving device through repeated total reflection inside of the light guide member. Accordingly, the condensing efficiency for the light emitted by the stimulable phosphor sheet can be kept high.
A technique for improving the condensing efficiency even further in the system using a light guide member has been proposed in, for example, U.S. Pat. No. 4,680,473. With the proposed technique, a condensing mirror is located on the side opposite to the light guide member with the main scanning line on a stimulable phosphor sheet intervening between the condensing mirror and the light guide member. Light, which is emitted by the stimulable phosphor sheet and which is not directly directed to the light input end face of the bight guide member, is reflected by the condensing mirror and guided to light input end face of the light guide member.
In cases where the light guide member is provided or in cases where the light guide member and the condensing mirror are provided, stimulating rays, which have been reflected from the stimulable phosphor sheet, are again reflected by the light guide member or by the condensing mirror and the light guide member. The thus reflected stimulating rays again impinge upon the stimulable phosphor sheet and stimulate a point on the stimulable phosphor sheet other than the point, which is being scanned. Light emitted by the point on the stimulable phosphor sheet other than the point, which is being scanned, is detected as flare light. If much flare light is detected, the sharpness of the image, which is obtained from the image read-out operation, cannot be kept good.
As described above, the problems with regard to the flare light occur when the light emitted by the stimulable phosphor sheet is detected. The same problems as those described above occur also when a light guide member is used or when a light guide member and a condensing mirror are used in a photo detecting apparatus, wherein light, which is emitted or reflected by a scanning surface when the scanning surface is scanned with a light beam, is detected by a photodetector.